The present invention generally relates to a method and system for improving the performance of a gas turbine engine by varying the combustor airflow using non-mechanical means.
The conventional gas turbine combustor, as used in a gas turbine power generating system, requires a mixture of fuel and air which is ignited and combusted uniformly. Generally, the fuel injected from a fuel nozzle into the inner tube of the combustor is mixed with air for combustion, fed under pressure from the air duct, ignited by a spark plug and combusted. The gas that results is lowered to a predetermined turbine inlet temperature by the addition of cooling air and dilutent air, then injected through a turbine nozzle into a gas turbine.
The supply of air from the engine compressor to the combustion chamber is usually divided into two main flows: combustion and dilution flow. Combustion flow is usually fed upstream to primary and secondary zones. Dilution air is usually fed through a series of apertures in the downstream end of the chamber to cool the hot gases from the combustion zone to a temperature acceptable to the turbine and such that the gases are at a uniform temperature.
It is well known within the art that exhaust gases produced by combusting hydrocarbon fuels can contribute to atmospheric pollution. This occurrence is attributed to the development of a localized high temperature zone, which can exceed 2,000xc2x0 C. Exhaust gases typically contain many undesirable pollutants such as nitric oxide (NO) and nitrogen dioxide (NO2), which are frequently grouped together as Nitrogen Oxides (NOx), unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot.
Several methods are known in the art to decrease NOx emissions. For example, the formation of NOx can be reduced by burning a low nitrogen or nitrogen-free fuel. The formation of NOx can be reduced by operating under uniformly fuel-lean conditions, such as by using a lean diffusion flame or a lean premixed/prevaporized (LPP) system. The excess air used to achieve fuel-lean combustion acts as a diluent to lower flame temperatures, thereby reducing the amount of thermal NOx formed. Prompt NOx can also be reduced by operating under fuel-lean conditions.
Generally CO pollutants are formed at low power conditions and NOx pollutants are formed at high power conditions. This leads to the problem that methods of reducing CO tend to increase amounts of NOx created. Likewise, efforts to decrease NOx emissions generally cause an increase in CO conditions.
For example, a convenient way of reducing the maximum temperature and therefore the formation of NOx, a problem which is acute in high pressure ratio engines where the combustion temperature is increased by the higher temperature of the supply air, is to operate the combustion primary section at an off-stoichiometric mixture strength, since the formation of NOx is at a maximum when the air and fuel mixture is stoichiometric and decreases rapidly as the mixture is richened or weakened. Thus NOx can be reduced provided the equivalence ratio is greater than 1.2 (fuel rich) or less than 0.8 (fuel weak).
However, this solution leads to a further problem, because when the engine operates at a part load condition there is a tendency for the equivalence ratio and the compressor delivery air temperature to drop causing the emission of large quantities of CO and the likelihood of combustion instability. A solution to this problem is to control the air to fuel ratios in the flame tube to suit the varying operating conditions. In this way the best compromise between combustion efficiency and the production of exhaust gas pollutants, and exit temperature can be achieved. This desirable state is achievable, either by the method known as fuel staging or by the method of regionally controlling the division of the air.
U.S. Pat. No. 4,062,182 to Fehler et al, herein incorporated by reference, provides a good example of the practice of fuel staging. Fuel staging involves the placing of air: fuel mixtures of known ratios into selected portions of the combustion zone. It usually involves at least two independently controllable stages of fuel injectors and a corresponding number of groups of air and fuel inlets into the combustion zone for the passage of two or more independent air and fuel mixture flows. By this method the combustion zone can be divided up into a first portion in which the air: fuel mixture is fuel rich as required for good ignition, the engine starting cycle and the lean mixture stalling limit and a second portion in which the air and fuel mixture is air rich to provide the lower combustion temperature. The net result of this arrangement is to achieve the desired lower bulk combustion temperatures and so keep the NOx emissions at an acceptable level whilst still being able to operate at low power conditions without generating substantial quantities of CO. The disadvantage of such an arrangement is the complexity of the fuel supply, in that at least two separately controllable fuel supply systems are required each having to be able to be assembled and disassembled with respect to the combustor and requiring relatively large apertures in the engine casing for such purposes. Any mechanical means of controlling the air supply is inherently fraught with problems associated with clogging and added complexity.
As can be seen, there is a need for a system and method for controlling and modulating the airflow to a lean premix combustor that allows for reduced power and reduced emissions.
The present invention is directed to a fluidic system and method for controlling the airflow to a lean premix combustor. The system and method as disclosed allows for reduced power, which results in reduced emissions, while at the same time providing excellent performance. This is accomplished through the use of fluidic means, which do not have the problems associated with hot moving parts of prior art mechanical means.
In one aspect of the invention is a gas turbine engine combustion system for varying the fuel/air ratio, comprising an inner shell with a premix section, a primary section, and a dilution section. There may be an outer shell accommodating the inner shell, a heat shield interposed between the outer shell and the inner shell, wherein the heat shield has a multiplicity of small holes. A casing may be attached to the heat shield and surrounding each of multiplicity of small holes. There may be a passage between the inner shell and the heat shield. The dilution section of the inner shell may have at least two dilution holes, which allow air to flow into said dilution section.
In another aspect of the present invention, a gas turbine engine combustion system for varying the fuel/air ratio is disclosed comprising an inner shell with a premix section, a primary section, and a dilution section. An outer shell accommodating the inner shell, and a heat shield may be interposed between the outer shell and the inner shell. The heat shield may have at least one set of small holes. An annular casing may be attached to the heat shield and surrounding at least one set of small holes. There may also be an annular passage between the inner shell and the heat shield and at least one set of dilution holes in the dilution section of said inner shell, wherein said at least one set of small holes in the heat shield are opposite and downstream from at least one set of dilution holes.
In another aspect of the present invention, a combustion chamber for varying air flow within a lean premix combustor engine is disclosed comprising an inner shell with a premix section, a primary section and a dilution section, an outer shell accommodating the inner shell and a heat shield interposed between the outer shell and the inner shell. The heat shield may have a first set of small holes and a second set of small holes. The first set of small holes may be parallel to the second set of small holes. There may also be an annular casing attached to the heat shield and surrounding the first set of small holes. There may also be an annular casing attached to the heat shield and surrounding the second set of small holes. An annular passage between the inner shell and the heat shield is also disclosed. A first set of dilution holes in the dilution section of the inner shell may be opposite and downstream from the first set of small holes. A second set of dilution holes in the dilution section of the inner shell may be opposite and downstream from the second set of small holes. Turbulation ribs may be attached to the outside of the inner shell and downstream from the dilution holes.
In yet another aspect of the present invention, there is disclosed a method of modulating the airflow to effectuate a desired fuel/air ratio in a lean premix combustor. This method may comprises the steps of reducing power, which reduces fuel flow into the combustor, reducing air into a primary section, cooling an inner shell by convective cooling and increasing air into a dilution section by introducing at least one stream of air through a multiplicity of openings in a heat shield that is interposed between and inner shell and an outer liner. The openings in the heat shield may be downstream from a multiplicity of dilution holes in an inner shell. At least one stream of air causes a local boundary layer separation to encourage blockage just below the said multiplicity of dilution holes so as to encourage air to enter said multiplicity of dilution holes.
In yet another aspect of the present invention, a method of modulating the airflow to effectuate a desired fuel/air ratio in a lean premix combustor is disclosed comprising the steps of reducing power, which reduces fuel flow Into the combustor; reducing air into a primary section, cooling an inner shell by convective cooling and increasing air into a dilution section by introducing at least one stream of air through a multiplicity of openings in a heat shield that is interposed between and inner shell and an outer liner. The openings in the heat shield are downstream from a multiplicity of dilution holes in an inner shell and at least one stream of air causes a local boundary layer separation to encourage blockage just below the multiplicity of dilution holes so as to encourage air to enter dilution holes.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.